This invention relates to a PNPN semiconductor device high in both capability and operating frequency.
High capability thyristors previously used with inverter apparatus have had the operating frequency limited to 5 kilohertz. Recently there are being demanded thyristors having higher operating frequency, for example the frequency of 10 kilohertz. In order to increase the operating frequency of thyristors to 10 kilohertz or higher, those thyristors are required to have the following performance:
1. The turn-off time should be of 10 microseconds or less.
2. The switching loss should be low. More specifically, the entire region should be brought into the conducting state within a short time interval with a low voltage drop in the conducting state.
High frequency thyristors of the conventional construction have included the main thyristor section and the auxiliary thrysitor section and manufactured by doping the silicon substrate thereof with a heavy metal such as gold (Au) or the like in order to decrease the lifetime of carriers to shorten the turn-off time. In such thyristors it is well known that the application of the gate trigger pulse thereto causes first the conduction of the auxiliary thyristor section followed by the conduction of the main thyristor section. Thus those thyristors have exhibited a shorter time interval to conduct the entire region thereof as compared with conventional thyristors without the auxiliary thyristor section.
However, in the process of manufacturing the thyristors as above outlined, a decrease in lifetime of carriers due to the diffusion of a heavy metal has brought about an increase in voltage drop during the conduction on the one hand and an increase in leakage current making it difficult to provide the thyristors high in OFF-state voltage on the other hand. Therefore it has been difficult to simultaneously meet the requirements (1) and (2) as above described.
Also one of known methods of decreasing the turn-off time is to interrupt the principal current through high frequency thyristors by inverting a voltage across the anode and cathode electrodes thereof followed by the application of a reverse voltage across both electrodes whereby the turn-off time is materially decreased. Since that method can decrease the turn-off time without an decrease in lifetime of carriers, it is extremely advantageous in that high frequency thyristors meeting the requirements (1) and (2) as above described are manufactured. However it is required to apply a sufficient magnitude of a reverse voltage across the junction involved. As a result, even if the method as above described would be applied directly to the conventional structure of high frequency thyristors, then it is impossible to apply a sufficient magnitude of a reverse voltage across the junction leading to the unsatisfactory results.